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Author Topic: Can you start fires with moonlight?  (Read 20435 times)

Offline burning

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Can you start fires with moonlight?
« Reply #25 on: 02/11/2011 02:09:44 »
In this case, the source of the REFLECTED light is the sun, and not the moon.

Yes, but's really no such thing as "reflected". Solar energy was absorbed by the Moon and energy was emitted by the Moon.

This is at best a dangerously misleading word game.  The are two different processes at work.  The one called reflection will produce a very different spectrum than that produced by the process called absorption and emission.  The reflected spectrum is proportional to the incident spectrum.  The emitted spectrum is proportional to the black body spectrum at the temperature of the object.*

Now, I suspect (but have not done the work to prove) that moonlight can't be focused enough, but the temperature of the moon is unlikely to be a significant limiting factor, because the spectrum of moonlight is dominated by reflected sunlight, not thermal radiation from the moon.  As Bored Chemist points out, the specular nature of the reflection is important.  The incident sunlight is getting more or less uniformly spread out over 2 π steradians, which is a considerable dilution.




* OK, there is a complication that the absorbtivity (and hence the reflectivity) of the object is a function of frequency, so it's not really simple proportionality.  However, since the absorbtivity function equals 1 minus the reflectivity function, and these two related functions are applied to two unrelated specta, the point still remains that they are measurably distinct processes.
 

Offline Geezer

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Can you start fires with moonlight?
« Reply #26 on: 02/11/2011 03:32:58 »
This is at best a dangerously misleading word game. are applied to two unrelated specta, the point still remains that they are measurably distinct processes.

Doesn't the question boil down to energy transmission and absorption. I could be wrong, but I don't think photons simply "bounce off" mirrors.
 

Offline burning

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Can you start fires with moonlight?
« Reply #27 on: 02/11/2011 04:40:12 »
This is at best a dangerously misleading word game. are applied to two unrelated specta, the point still remains that they are measurably distinct processes.

Doesn't the question boil down to energy transmission and absorption. I could be wrong, but I don't think photons simply "bounce off" mirrors.

For the moment, don't think about what's happening at the level of a single photon, and let's consider the two extreme behaviors.

An ideal absorber, i.e. a "black body," absorbs all light in the sense that any spectral information about light incident on it is lost.  The light emitted from the surface of the black body is a function of its temperature only.  True, the temperature of the body will change if the net power absorbed does not equal the net power radiated. But whether we shine 5 Watts of infrared, 5 Watts of visible, 5 Watts of UV, or 5 Watts distributed over a broad band, it doesn't matter; the spectrum coming from the surface of the body will be the same.

An ideal reflector, a "white body", can be said to not absorb, because the light coming from its surface completely preserves the spectral information of the light incident on its surface.  Indeed, if there is no light incident on it, there is also no light emitted from it.

OK, now realistic objects are neither ideal absorbers nor ideal reflectors.  But they can be understood as some combination of the two (neglecting the third process of transmittance, which is a safe omission in the case of the moon).  Some fraction of the light incident will be absorbed and thermalized; this light will potentially affect the temperature, which in turn determines what light is radiated from the surface.  Some fraction will come back out spectrally unchanged from how it was on the way in.

Now if we consider what's going on at the photon level, are the photons coming out that preserve spectral information of the incident light the same actual photons that were incident in the first place?  Probably not, but so what?  Why should detailed knowledge of what is going on at the particle level invalidate the use of the word "reflection" for the process of redirecting light with its spectral information intact?  Particularly, why should we insist on giving it the same name as the process that removes all spectral information of the incident light?
 

Offline Geezer

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Can you start fires with moonlight?
« Reply #28 on: 02/11/2011 05:02:03 »
OK, but I suppose we should try to get back to the original question and try to exlain the answer.
 

Offline Bored chemist

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Can you start fires with moonlight?
« Reply #29 on: 02/11/2011 06:58:18 »
"But they can be understood as some combination of the two (neglecting the third process of transmittance, which is a safe omission in the case of the moon)."
Not if we are interested in the spectrum of the light reflected.
http://web.eng.fiu.edu/~godavart/BME-Optics/Kubelka-Munk-Theory.pdf
 

Offline burning

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Can you start fires with moonlight?
« Reply #30 on: 04/11/2011 03:02:26 »
OK, I've been thinking about this for a couple days, and I've finally got it.  The quantity power per unit area per unit solid angle* is one that can only decrease as light goes through any optical system.  If the system consisted of only perfect mirrors and perfectly transparent lenses, the quantity would be conserved.  This basically boils down to being a consequence of conservation of energy.

Now the power per unit area from the Sun at the surface of the Moon is essentially the same as at the surface of Earth, and we know that it isn't enough to start a fire.  The moon is a diffuse reflector, so even if it were a perfect diffuse reflector, the power per area per solid angle coming from the moon would be the incident power per area divided by 2 π steradians.

No matter what optical system we might devise to focus this light, it is impossible to devise one that subtended more than 4 π steradians from a point on the surface of the target paper.  Which means that the absolute theoretical maximum power per unit area of focused moonlight would only be twice the power per unit area of sunlight at the Earth.  I'm pretty sure that only doubling the power per unit area of sunlight would not be enough to start a fire, and any realistic value for reflected moonlight is going to be less than this amount.



*Wikipedia calls this the radiance.  In my graduate optics class it was called the intensity.  Radiance is probably better, since a lot of different quantities get saddled with the name "intensity" by someone or other.
 

Offline Geezer

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Can you start fires with moonlight?
« Reply #31 on: 04/11/2011 05:46:35 »
and we know that it isn't enough to start a fire.

I suspect you didn't really mean to say that :D
 

Offline willyp00

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Can you start fires with moonlight?
« Reply #32 on: 04/11/2011 09:14:04 »
Luke McNeill  asked the Naked Scientists:
   
Dear Nakeds

Everybody knows you can use a magnifying glass to focus sunlight to burn things, and the larger the lens, the more intense the heat. How large of a lens would you need to use to focus light from the full moon to burn paper?

Thanks,
Luke McNeill
Arlington, MA, USA

What do you think?
If the lens were big enough and the conditions were perfect, yes. What is the directed power of reflected moonlight at us? There must be some langleys or watts out there
 

Offline willyp00

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Can you start fires with moonlight?
« Reply #33 on: 04/11/2011 09:18:11 »
In this case, the source of the REFLECTED light is the sun, and not the moon.

Yes, but's really no such thing as "reflected". Solar energy was absorbed by the Moon and energy was emitted by the Moon.

This is at best a dangerously misleading word game.  The are two different processes at work.  The one called reflection will produce a very different spectrum than that produced by the process called absorption and emission.  The reflected spectrum is proportional to the incident spectrum.  The emitted spectrum is proportional to the black body spectrum at the temperature of the object.*

Now, I suspect (but have not done the work to prove) that moonlight can't be focused enough, but the temperature of the moon is unlikely to be a significant limiting factor, because the spectrum of moonlight is dominated by reflected sunlight, not thermal radiation from the moon.  As Bored Chemist points out, the specular nature of the reflection is important.  The incident sunlight is getting more or less uniformly spread out over 2 π steradians, which is a considerable dilution.




* OK, there is a complication that the absorbtivity (and hence the reflectivity) of the object is a function of frequency, so it's not really simple proportionality.  However, since the absorbtivity function equals 1 minus the reflectivity function, and these two related functions are applied to two unrelated specta, the point still remains that they are measurably distinct processes.
Reflection is absorption and emission.
 

Offline Bored chemist

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Can you start fires with moonlight?
« Reply #34 on: 04/11/2011 11:03:32 »
Luke McNeill  asked the Naked Scientists:
   
Dear Nakeds

Everybody knows you can use a magnifying glass to focus sunlight to burn things, and the larger the lens, the more intense the heat. How large of a lens would you need to use to focus light from the full moon to burn paper?

Thanks,
Luke McNeill
Arlington, MA, USA

What do you think?
If the lens were big enough and the conditions were perfect, yes. What is the directed power of reflected moonlight at us? There must be some langleys or watts out there

Actually, someone (under the name of Burning) has just shown that, even if the conditions were perfect, the answer is no.
 

Offline syhprum

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Can you start fires with moonlight?
« Reply #35 on: 04/11/2011 12:16:03 »
There is no doubt that it cannot be done with imaging optics but the jury is still out as it whether it can be done with non imaging optics
 

Offline JP

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Can you start fires with moonlight?
« Reply #36 on: 04/11/2011 13:34:18 »
*Wikipedia calls this the radiance.  In my graduate optics class it was called the intensity.  Radiance is probably better, since a lot of different quantities get saddled with the name "intensity" by someone or other.

The radiance is power/(unit area*unit solid angle)

Radiant intensity is power/(unit solid angle)

Irradiance is power/(unit area).

Intensity is most often associated with irradiance, since you're measuring energy incident upon a pixel (a tiny unit of area) integrated over some time.

But good explanation.     
 

Offline Geezer

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Can you start fires with moonlight?
« Reply #37 on: 05/11/2011 05:12:00 »
Intensity is most often associated with irradiance, since you're measuring energy incident upon a pixel (a tiny unit of area) integrated over some time.   

Very nice, but still a load of old moonshine.

A simple magnifying glass could easily start a fire on the Moon (if there was some oxygen) using the light from the Sun. The Earth and the Moon are essentially the same distance from the Sun.

The reason the light from the Moon cannot start a fire on Earth is simply because the Moon has "soaked up" all the IR from the Sun. Basically, the Moon is a lousy reflector. If it was not, it would look like a second Sun.
« Last Edit: 05/11/2011 05:35:28 by Geezer »
 

Offline syhprum

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Can you start fires with moonlight?
« Reply #38 on: 05/11/2011 08:15:13 »
Has anyone checked the output of a solar panel when illuminated by Moonlight I would expect it to be pretty low
 

Offline burning

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Can you start fires with moonlight?
« Reply #39 on: 05/11/2011 15:23:55 »
and we know that it isn't enough to start a fire.

I suspect you didn't really mean to say that :D

That value of watts per square meter is not enough to start a fire.  Have you ever put a piece of paper out under the sun and see it ignite?  Yes, you can magnify the direct light of the sun and start a fire, but then you wouldn't have the incident power per unit area, you would have a greater one, due to the magnification.


(Edit: Just replacing quote I was responding to that got lost in pre-post editing.)
« Last Edit: 05/11/2011 15:54:37 by burning »
 

Offline burning

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Can you start fires with moonlight?
« Reply #40 on: 05/11/2011 15:54:10 »
Intensity is most often associated with irradiance, since you're measuring energy incident upon a pixel (a tiny unit of area) integrated over some time.   

Very nice, but still a load of old moonshine.

A simple magnifying glass could easily start a fire on the Moon (if there was some oxygen) using the light from the Sun. The Earth and the Moon are essentially the same distance from the Sun.

The reason the light from the Moon cannot start a fire on Earth is simply because the Moon has "soaked up" all the IR from the Sun. Basically, the Moon is a lousy reflector. If it was not, it would look like a second Sun.

If the Moon were a perfect specular reflector (and flat and oriented correctly) it would look like a second Sun.  If it were a perfect diffuse reflector, it most certainly would not.

For an observer on Earth or the Moon, the sun subtends a very small solid angle (about 6 x 10-5 steradians).  After reflecting off a perfect diffuse reflector located where the Moon is, that light would be spread out over 2π steradians, i.e. a reduction in radiance (power per unit area per unit solid angle) of five orders of magnitude.  Since the Moon is coincidentally about the same solid angle as the Sun when viewed from the Earth, that means that the unfocused power per unit area from the Moon is at best (assuming perfect reflectivity across the whole spectrum) five orders of magnitude less than the unfocused power per unit area from the Sun.

Now, it may be that if we replaced the moon with a flat specular reflector, but an imperfect one which had the same spectral reflectivity of the moon, that we would indeed lose too much power at frequencies that paper absorbs efficiently to be able to focus the moonlight enough to start a fire.  I can't speak for anyone else, but it was never my intention to say that diffuse nature of reflection from the Moon was the only factor.  I do assert, however, that understanding the consequences of diffuse reflection is sufficient in and of itself to answer the question with a No.
 

Offline Bored chemist

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Can you start fires with moonlight?
« Reply #41 on: 05/11/2011 16:08:18 »
The amount of radiation absorbed (rather than reflected) by the moon is a bit of a red herring, It heats up the moon until the moon re-radiates it as pretty much black body radiation so, with the right sort of lens (or a good mirror) not much energy is lost. The big issue is, as Burning says, that the light is spread out so much.
 

Offline syhprum

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Can you start fires with moonlight?
« Reply #42 on: 05/11/2011 16:19:25 »
The ignition temperature of paper is reputed to be 451°F (233°C) has this been verified ?
 

Offline JP

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« Reply #43 on: 05/11/2011 16:27:27 »
The ignition temperature of paper is reputed to be 451°F (233°C) has this been verified ?
Yes.
 

Offline Bored chemist

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« Reply #44 on: 05/11/2011 17:11:04 »
The ignition temperature of paper is reputed to be 451°F (233°C) has this been verified ?
Yes.
No.
I always rather doubted this so I checked.
I got some lead, melted it and heated the liquid to about 400C (as measured with a thermocouple thermometer).
I then poured it into a folded  piece of paper.
The paper didn't even smoke much.
My thermometer might not be accurate, but the melting point of lead is about 327C or 621 F so this paper was certainly hotter than that.
 

Offline JP

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« Last Edit: 05/11/2011 18:12:56 by JP »
 

Offline syhprum

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Can you start fires with moonlight?
« Reply #46 on: 05/11/2011 19:21:21 »
I am a little surprised that Ray Bradbury did not do some research before settling on a title for his novel, it is obvious to most people that 451°F or even 451°C is much too low. 
 

Offline JP

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Can you start fires with moonlight?
« Reply #47 on: 05/11/2011 19:27:17 »
I am a little surprised that Ray Bradbury did not do some research before settling on a title for his novel, it is obvious to most people that 451°F or even 451°C is much too low. 

Check the link above.  451 C is not too low, although F is.
 

Offline Geezer

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Can you start fires with moonlight?
« Reply #48 on: 06/11/2011 00:30:44 »
The amount of radiation absorbed (rather than reflected) by the moon is a bit of a red herring, It heats up the moon until the moon re-radiates it as pretty much black body radiation so, with the right sort of lens (or a good mirror) not much energy is lost. The big issue is, as Burning says, that the light is spread out so much.

I suppose that's true, although it probably takes a little time to warm up after sunrise.
 

Offline Geezer

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« Reply #49 on: 06/11/2011 00:51:05 »
and we know that it isn't enough to start a fire.

I suspect you didn't really mean to say that :D

That value of watts per square meter is not enough to start a fire.  Have you ever put a piece of paper out under the sun and see it ignite?  Yes, you can magnify the direct light of the sun and start a fire, but then you wouldn't have the incident power per unit area, you would have a greater one, due to the magnification.


(Edit: Just replacing quote I was responding to that got lost in pre-post editing.)

Right, so it's just a matter of magnification. In that case, unless the spectrum of moonlight does not allow it, it should not be impossible with moonlight either, although it may be completely impractical.

Anybody know what the spectrum of moonlight looks like? I was going to say it looks as if it's not emitting much towards the red end, but, on reflection, I think that's not right. I imagine the light from the Sun must appear white on the Moon.
« Last Edit: 06/11/2011 01:46:41 by Geezer »
 

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